Bent nitrosyls

Synthesis of this compound from a 15N labelled source revealed that the i4N and 15N were equally distributed between the apical bent nitrosyl (NO-) and equatorial linear nitrosyl (NO+) ... 

Coordination of NO to the divalent tetrasulfonated phthalocyanine complex [Co(TSPc)]4 results in a complex formally represented as [(NO )Coin(TSPc)]4 kf= 142M-1s-1, KA 3.0 x 105 M-1). When adsorbed to a glassy carbon electrode, [Co(TSPc)]4- catalyzes the oxidation and reduction of NO with catalytic currents detectable even at nanomolar concentrations. Electrochemistry of the same complex in surfactant films has also been studied.905 Bent nitrosyl complexes of the paramagnetic trivalent tropocoronand complex Co(NO)(TC) ((189), R = NO) have also been reported.849... 

The first well-characterized example of a bent nitrosyl ligand was that found in a derivative of Vaska s complex ... 

The product is square pyramidal with a bent nitrosyl ligand (Z. Ir—N—O - 124 ) at ihe apical position (Fig. 15.15a).44 Other complexes with a bent M—NO have been found, including the remarkable example [Ru[Pg.863]

Fig. 15.15 Complexes containing bent nitrosyl groups la, llrlPPhjJjlCOllNOJCI] anil (b) lRu(PPh3) (NOhC,r. Phenyl groups have been omitted for clarity, disiances are in picomeiers. [Structure (a) from Hodgson, D. J., Payne, N. C. McGinnety, J. A. Pearson, R. G. Ibers, J. A. J. Am. Chem. See. 1968, 90, 4486-4488. Structure

The structural studies of nitrosyls have shown to date that the bent nitrosyl ligand invariably occurs at the apical position of a square based pyramid or a distorted octahedron in which the metal ion configuration assuming NO- coordination is d6. Despite numerous electronic structural descriptions (168, 169,198-200), it is not totally clear why fully bent nitrosyls with M—N—O bond angles of 120° have not been found in other geometries such as the square plane and the trigonal bipyramid. 

The NO+ and NO- modes of coordination differ by two electrons in terms of formal charge. Interconversion of these two bonding modes becomes feasible when the bound metal ion possesses two complementary oxidation states. It has been proposed that this interconversion, which corresponds to an intramolecular redox reaction, represents a unique and facile way to achieve coordinative unsaturation at the metal center with the nitrosyl acting as an electron pair reservoir (201). Interconversion of linear and bent nitrosyls has been reported in the unusual complex Ru(NO)2C1-(PPh3)2+ that possesses one linear and one bent nitrosyl, structure (39) (202). 

In this reaction the nitrosyl ligand bends, undergoing a 2 e reduction, rather than form a 20 e complex. This reaction can also serve to activate the nitrosyl ligand. Whereas the linear nitrosyl may be unreactive if vNO is sufficiently low, the bent nitrosyl is reactive to electrophiles. 

Electrophilic attack by H+ on bent nitrosyl ligands leads to reduction (190, 205, 216). Complexes containing HNO, NHOH, and NH2OH have all been prepared in this manner, raising the possibility, as yet unrealized, of a catalytic reduction of NO to hydroxylamine. The simplest and best characterized of these reactions is (91) reported by Enemark et al. (205). 

The electrophilic attack of nitric oxide on a bent nitrosyl is now realized to be the path by which hyponitrite-bridged Co species are formed. Reaction (93) was known since the time of Werner (217), but the black and red isomers of [Co(NO)(NH3)5]2+ obtained from this reaction defied definitive characterization for many years. It has now been established that the black isomer is a mononuclear, octahedral complex of Co(III) and NO- (218) while the red isomer is a hyponitrite bridged system containing two Co(III) ions (219). 

In (95), the initial formation of the bent nitrosyl species [Co(NO)(en)2Cl]+ (220) is followed by electrophilic attack of free NO and then by reaction with a second NO molecule leading to products. Since the nitro complex produced in this reaction has the same stereochemistry as the reactant Co111—NO- species, it has been proposed that the Co—N(nitrosyl) bond remains intact throughout the reaction sequence, thus requiring free NO to attack via its oxygen end (193). This proposal requires further proof. 

All of the reactions outlined above show by example that bent nitrosyls are activated to electrophilic attack. 

The bent nitrosyl may then act as a nucleophile on the benzyl halide, leading to PhCH=NOH and PhCN, the latter arising from the former by dehydration. 

A parallel situation appears to obtain for the mixed allyl nitrosyl complex Ru(NO)(C3H5)L2 prepared by Schoonover and Eisenberg (231). This complex which is coordinatively saturated (NO+ and rf -allyl), forms a CO adduct which is assigned a bent nitrosyl structure (231). Further reaction under CO leads to the formation of Ru(CO)3L2 with the possible elimination of acrolein oxime. The coupling of the allyl and nitrosyl ligands can be viewed in this case as nucleophilic attack of NO- on an f/3-allyl species. Unlike in reaction (110), both of the moieties to be coupled lie within the same coordination sphere. The significance of these results is that it lends viability to the notion embodied in (109) in which a migratory insertion of nitrosyl occurs as NO-. 

In this catalysis a surface allyl species is formed which combines with NO to form 3-nitrosopropene. Tautomerism and dehydration then lead to the acrylonitrile product. The coupling of the allyl and nitrosyl ligands in Ru(NO)(CO)(C3H5)L2 (231), via (112) represents a key step in the proposed reaction sequence and suggests the necessity of a bent nitrosyl, at least in a discrete complex case. 

This structure should be contrasted with an alternative formulation of this species as a complex containing one linear and one bent nitrosyl which is suggested in (110) for the Ru analog. The dinitrogen dioxide ligand differs... 

The new peak which grows in at 1835 cm"1 is assigned to the NO stretch in the excited state molecule. Its energy is reduced by 105 cm- from that of the NO stretch in the electronic ground state. The lower frequency is expected for the bent nitrosyl where the formal bond order is reduced from three to two. The decrease is similar in magnitude to that observed in RuCl(N0)2(PPh3)2 where both a linear and a bent nitrosyl is observed (24). 

Several mixed carbonyl-nitrosyls having the 18-electron count have been prepared from [Co(NO)(CO)3] or [Co(NO)2Br]2 (Table 11). All are four coordinate and probably tetrahedral, although the low vNO for K3[Co(NO)(CN)3] (1485 cm-1) may imply square-planar coordination with a bent nitrosyl. Rates of substitution in poorly coordinating solvents (THF, xylene equations 11 and 12) follow the rate expression kobs = k, + k,[L], with kt and k2 being attributed to dissociative paths involving the sovlent and ligand respectively.94 For L—L chelates formation of the monoden-tate species is rate determining.95... 

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